This application is a continuation-in-part of U.S. patent application Ser. No. 052,700 entitled Secondary Electron Detector for Use in a Gaseous Atmosphere, filed May 21, 1987, now Pat. No. 4,785,182 and also U.S. patent application No. 158,208, entitled Integrated Electron Optical/ Differential Pumping/ Imaging Signal Detection System for an Environmental Scanning Electron Microscope, filed Feb. 19, 1988 now U.S. Pat. No. 4,823,006.
This invention relates to the field of environmental scanning electron microscopes (ESEM), and more particularly to an environmental scanning electron microscope having a practical multi-electrode configuration of a gaseous detector device for the definition of detection volume and signal separation in the ESEM.
As background, the advantages of the ESEM over the standard scanning electron microscope (SEM) lie in its ability to produce high-resolution electron images of moist or nonconductive specimens (e.g., biological materials, plastics, ceramics, fibers) which are extremely difficult to image in the usual vacuum environment of the SEM. The ESEM allows the specimen to be maintained in its "natural" state, without subjecting it to the distortions caused by drying, freezing, or vacuum coating normally required for high-vacuum electron beam observation. Also, the relatively high gas pressure easily tolerated in the ESEM specimen chamber acts effectively to dissipate the surface charge that would normally build up on a nonconductive specimen, blocking high quality image acquisition. The ESEM also permits direct, real-time observation of liquid transport, chemical reaction, solution, hydration, crystallization, and other processes occurring at relatively high vapor pressures, far above those that can be permitted in the normal SEM specimen chamber.
This technological advance in imaging capability opens up a previously hidden world of microscopic phenomena to investigators in a wide spectrum of disciplines, including, but not limited to, medicine, biology, food and drug technology, geology, composite materials, textiles, semiconductors and forensics, in short, any field involving specimens which are difficult to image with the standard SEM.
A previous problem, however, encountered in utilizing previous ESEM designs was the inability of the gaseous detector device to separate the various signals emitted by the electron beam impinging upon the specimen, such as low energy secondary electrons, higher energy backscattered electrons, low-angle reflected electrons and high-angle reflected electrons. As a result thereof, this invention provides a practical multi-electrode configuration for a gaseous detector device which allows for the definition of detector volumes and signals separation in the ESEM. As such, this ESEM provides for better imaging capabilities.
Furthermore, in previous ESEM's, an electron detector was placed integral with the pressure limiting aperture. The positively biased detector electrode/pressure limiting aperture and the specimen holder (at ground) form a parallel-plate capacitor wherein the electrical potential drops uniformly with the distance across this gap, that is, free electrons experience a uniform acceleration toward the positive electrode regardless of their position in the gap. It has been found however that if an electrically isolated screen or grid is positioned between the specimen and the detector electrode, significant improvements in the collection of signals in the ESEM results. By applying appropriately scaled voltages to the grid (and/or the specimen holder) it is possible to divide the collection region or "detection volume" into two zones of independent field strengths one above and one below the grid. The grid thus become a control element in the electron detector system.